The present invention relates generally to a method for semiconductor wafer manufacturing, and, more particularly, to a method for forming a solid solution alloy crystal.
Silicon is widely used for fabricating semiconductor devices such as integrated circuits, discrete devices, and sensors. In a typical integrated circuit fabrication process, a wafer of crystal material may be exposed to several processing steps such as doping, etching, or implanting to form an array of resulting integrated circuits. The resulting integrated circuits are then individually separated from the wafer and packaged. Typical integrated circuits include several transistors interconnected together.
More recently, the dimensions of semiconductor devices have been reduced such that more devices are formed per unit area on a single wafer. Accordingly, various processes have been developed which permit the forming of larger diameter wafers. This increases the number of circuits which can be placed on an individual wafer. Typically, large diameter wafers can be formed from single crystals that are produced using bulk seeded crystal growing processes to obtain specific crystallographic orientations.
One such technique is the floating-zone ("FZ") process. In this process, a narrow heater, such as a radio frequency single turn coil, surrounds a feed rod of polycrystalline material. The feed rod is positioned above a single seed crystal that has a desired crystallographic orientation. The heater melts the feed rod material to create a liquid of material from which a resulting single crystal is grown. A floating liquid zone is created, when the melted material contacts the single seed crystal. The feed rod polycrystalline material is then continuously converted into single crystal. The term single crystal refers to a crystal that is in the form of a monocrystalline material.
A single crystal can be doped by solid or gaseous sources during the growth process. The gaseous dopants may be diluted into an inert carrier gas and blown to the floating zone of the liquid silicon. The solid dopants may be incorporated into the feed rod. The dopants are dissolved into the liquid and then incorporated into the growing crystal.
One disadvantage to the FZ process is that solid alloy material, which may be used to increase the performance of the resulting circuit, may not be able to be added without disturbing the liquid zone. Thus, the resulting single crystal may only be used in a limited number of semiconductor applications.
Another known bulk crystal growing process is the Bridgman method. A closed tube configuration is employed in which a seed is placed at one end of the tube in a liquid. The liquid is cooled to cause the liquid to solidify as a single crystal. The resulting single crystal has a specific crystallographic orientation. One disadvantage to this technique is that unwanted stresses typically occur in the resulting single crystal.
Another known technique is the Czochralski ("CZ") or crystal pulling method. A seed attached to a shaft is lowered into a liquid pool. The shaft is then simultaneously rotated and raised (pulled) in a continuing sequence of precisely controlled steps. As the shaft is raised, crystallization occurs where the liquid contacts the seed to form a single crystal. The resulting single crystal is then sliced in a sawing process to produce individual wafers. Typically, a wafer may have a resulting diameter between 80-90% of the original diameter of the grown single crystal. This is because the crystal may be ground, lapped, and polished before the wafers are formed. Using known CZ methods, a single crystal having a length of about one meter and a diameter of about 80mm may take a day or more to produce.
Known CZ methods are adequate for single crystal growth, when the single crystal is formed from one chemical element or a compound. However, the growth of solid solution alloy crystals requires specific processing steps that increase the complexity of the growing process. In particular, an alloy crystal requires substantially lower growth rates, and therefore more processing time, to maintain suitable crystal growth. This is necessary to prevent the alloy crystal from degrading into a dendritic or polycrystalline form. For example, a processing step may take about forty times longer for an alloy crystal compared to an elemental or compound single crystal. However, in known CZ methods, the total processing time is fixed. As a result, both the length and diameter of a resulting alloy crystal is significantly limited.